1. Field of the Invention
The present invention relates generally to the field of gas sensors. More particularly, the present invention relates to semiconductor gas sensors made from wide bandgap materials such as gallium nitride (GaN) and silicon carbide (SiC) that are effective at providing continuous or discrete measure of gas levels resulting from degradation processes in insulating oil in oil-filled high-voltage electrical equipment.
2. Description of the Related Art
Gas sensors have been used in the detection of particular symptomatic gases in oil-filled electrical equipment. Faults in oil-filled transformers, for example, may include arcing (electrical), corona discharge (electrical), low energy sparking (electrical), severe overloading (electrical), pump motor failure (electrical and thermal) and overheating (electrical and thermal) in an insulation system. Faults may generate undesirable gases, such as hydrogen (H2), acetylene (C2H2), ethylene (C2H4), methane (CH4), ethane (C2H4), carbon monoxide (CO) and carbon dioxide (CO2). These fault conditions result in a malfunctioning transformer or may indicate an impending malfunction, which, if not corrected, may lead to failure of the transformer. A statistical correlation exists between transformer malfunction and fault gases generated by the transformer. Accordingly, if the accurate detection of potentially dangerous gases in a transformer is achieved, possible malfunction and failure of the transformer can be addressed and often avoided.
The principles described previously for oil filled transformers may also be applied to other pieces of oil filled equipment or facilities, in which high electrical fields or temperature oscillations cause the oil to break down into its potentially flammable constituents over time. One example of such equipment includes x-ray tubes used in medical applications. X-ray tubes supply x-rays used in medical assessments of bone or tissue structure. These tubes, much like transformers, use oil to both insulate and cool internal electrical components. Gas sensors fabricated from GaN or SiC would provide a non-intrusive method for maintaining such equipment regularly, minimizing down-time and avoiding catastrophic fault conditions.
With respect to hydrogen, power transformers expose insulating oil to high electric fields that break down the oil over time. Hydrogen gas and hydrogen bearing compounds are given off, indicating the need for preventative maintenance. If this need goes unheeded, it may lead to the build-up of flammable hydrogen gas in the system, which if ignited, may lead to catastrophic failure. Current detection systems for hydrogen include oil sampling and chromatographic analysis, single gas sensors and person-operated units. These conventional approaches are time consuming, expensive, offer incomplete information, and in some cases are only performed periodically throughout the year.
The ability of sensors to identify a target gas depends on several factors. These factors include the sensitivity of the sensor to other interfering gases and vapors, and a concentration of the target gas. The ability to resolve the target gas from other gases is called the selectivity. There are very few known sensors that are highly selective where a sensor has greater than about a tenfold difference in gas detection between sensing states and non-sensing states. Further, within these very few sensors there are even fewer that are relatively reliable to accurately detect individual gases.
Current semiconductor gas sensor technology may make use of Si/SiO2 as materials on which a gas is sensed. Others may make use of SnO2 or other oxides, however, in the case of SnO2, these devices typically require a heater to increase their temperature in excess of 200 deg C. in order to make them sensitive enough to be useful. While these sensors are mass producible, they often fail in outdoor environments where the temperature fluctuates. Temperature fluctuations may lead to drift in response to gaseous environments over time, which means that the change in electrical response to the same gas will differ over time, thus the sensor system will require temperature correction in order to track quantitative changes. Even small changes in a temperature range, such as about −40 to about 130 deg F., are enough to cause such drift over time. Drift is most noticeable in Si devices, making these devices ineffective in such ambient settings. In order to minimize drift, the Si-based sensors often require heating to a temperature of up to about 150 deg C. in order to return the sensors to nominal operating conditions. Despite the heating, drift over time still occurs due to surface states formed from oxides and other elements on the surface of the sensors.
U.S. Pat. Nos. 6,041,643, 6,155,100, 6,182,500 and 6,202,473 all issued to Stokes et al., incorporated herein by reference, describe a gas sensor for determining the presence of at least one gas in a gaseous environment. The gas sensor includes a semiconductor substrate, a thin insulator layer disposed on the semiconductor substrate, a catalytic metallic gate disposed on the thin insulator layer and a chemically modified layer disposed on the catalytic metal gate. The chemically modified layer includes a material that protects the sensor from corrosive gases and interference from at least one of foreign matter and water, alters at least one of surface chemical properties and surface physical properties of the sensor, and passes only a designated gas therethrough.
What is needed is a more robust material system for addressing material issues and eliminating drift. What is further needed is a high temperature, harsh environment capable gas sensor that outperforms conventional solid-state sensors that use semiconductor materials such as Si.